The present invention relates to the privacy of data used by modern computers and communication systems and more particularly to cryptography in which digital signals representing a clear text are changed to digital signals representing a ciphered text and vice versa.
Digital computers and communications systems are increasingly being used to store and transmit confidential data. The security and privacy of these systems calls for apparatus and method beyond those of the normal computing and communications functions of these systems and thereby increasing the overall systems costs. In the past, users of private data systems have gradually adopted a number of useful countermeasures to protect their systems and this activity has increased greatly since the passage of the Privacy Act of 1974 which requires that Federal use of computer files are now subject to stringent legislative constraints. The need for encryption is not only to satisfy the current legal requirements for privacy however but extends to protect data systems from criminal activities as well. Encryption is but one powerful countermeasure for combating a number of threats including the monitoring, misrouting, substitution, modification and injection of messages to and from terminals and their central processors. And, data files can also be safeguarded by using encryption techniques. This can all be seen in a number of publications including Refs [1]-[7].
When computer data is transmitted to a remotely located terminal, unauthorized access to it may be prohibited by using cryptographic encipherement in which the clear text data S is changed to enciphered text data S.sub.o related by S.sub.o = f(S, K) where K is the key and f is the cryptographic algorithm. Decipherement involves an inverse transformation yielding the clear text data sequence S from the cipher text data sequence S.sub.o. The two basic methods or schemes for ciphering data are the so called stream and block cipher schemes. In both schemes the encipherement and decipherement of data are under control of the key K. Both schemes have certain advantages and disadvantages but the block cipher is now generally accepted as being the least susceptible to determining (breaking) the key and in fact the proposed standard algorithm is for a block cipher. The apparent advantage of block over stream ciphers is that the one-to-one correspondence between the clear and cipher text bits is avoided. Each bit of the block encryption affects all bits of the block decryption thus making analysis extremely difficult. Stream ciphers have been discussed in a number of publications including Refs [3], [8], [9] while block ciphers are discussed in Refs [10]-[12].
There are known in the prior art a variety of algorithms which can be used to produce ciphers. As a consequence, the computer privacy problem has been burdened by the lack of standardization. To alleviate this problem, the National Bureau of Standards is considering the adoption of an algorithm as a Federal information processing standard. The proposed technique has been disclosed in Refs [13], [14]. No doubt whether the NBS proposed standard is finalized or not it will serve as an initializing activity for further work to say the least. In any case, the prior art cryptographic devices, both stream and block, have been limited by their software and hardware implementations and costs.
Stream and block cipher systems in the prior art are known using both software and hardware. Stream devices are the simplest and least expensive but suffer the least strength of their ciphers while block devices are the most expensive but offer the potential for the highest strength of their ciphers. And, software systems are slow when compared to hardware systems. In particular, hardware implementations of the prior art have been obtained in the form of shift registers. Such devices can be assembled from conventional medium scale integrated (MSI) circuit logic or can be designed in large scale integrated (LSI) form. Thus, the digital implementation of cipher systems requires the high speed storage the readout of data in a number of shift registers. However, shift registers are limited in length and speed, and many similar devices are needed if much data is to be stored, for example when seeking to strengthen the cipher.
In many applications the data must be recycled in time. This is accomplished in the prior art by recirculating data through shift registers; the advantage of recirculation being the saving in hardware over the cascading of many similar shift registers. Of particular interest to the present invention is the delay time compressor (DELTIC) which recirculates in a number of recirculations. The DELTIC has been used for implementing a variety of digital devices, for example the digital matched filter and correlator of my patent Ref [15] and the digital time compressor of my copending application Ref [16].
The prior cipher art using recirculating shift registers utilize non-DELTIC implementations. The system of the present invention on the other hand is charactered through its preferred use of the DELTIC circuit using shift registers and RAMs as delay elements and in this manner for providing new and improved stream and block cipher devices while significantly decreasing the weight, size, power consumption and cost for such devices. From this discussion it is clear that in the past, the digital impelementation of a cipher device has been accomplished using shift registers, non-recirculating and recirculating non-DELTIC, and for all practical purposes has not been successful for increasing the capacity and speed of operation of such devices beyond a certain limit determined by the technology of shift registers. Furthermore, the prior art of digital hardware implementations falls short when the size and cost of shift registers are considered in devices requiring high throughputs.
It is the purpose of the present invention to produce cipher devices capable of exceeding the practical capacity and speed of present digital stream and block cipher devices by at least one order of magnitude, at reduced size and cost.